Method 1 (tested by me and working):

1. “sudo apt-get install dontzap”
2. “sudo dontzap -d”

Method 2 (hasn’t been tested by me):

1. Add the following lines to your xorg.conf file, making sure that when you paste it, it is NOT using smart quotes: “sudo gksudo gedit /etc/X11/xorg.conf”

   Section “ServerFlags”

   Option “DontZap” “false”

   EndSection

2. you can use “right alt” + “printscreen” + “k”
   to do the same effect

Source 

Update: For Karmic, you won’t find that package, but there is a much simpler method:

Using GNOME
* Get to the System->Preferences->Keyboard menu.
* Select the “Layouts” tab and click on the “Layout Options” button.
* Then select “Key sequence to kill the X server” and enable “Control + Alt + Backspace”.

Source